Liquid hydrogen and liquid oxygen are amongst the most common propellants for space vehicles. Hydrogen and oxygen yield substantial amounts of energy per pound of propellant when combined.
These elements have an additional advantage of being extremely clean and safe propellants that when combined produce water. In addition to its use as a propellant, the liquid oxygen can be utilized in additional roles, such as in maintaining breathable atmospheric conditions within space vehicles, power generation from fuel cells. Oxygen and hydrogen also have the added benefit of being found in water, which may be found in sufficient quantities on the moon or on Mars in the from of ice, providing an indigenous propellant to be used to recharge the vehicle for return flights. By comparison, earth storable propellants, for instance combinations of hydrazine and dinitrogen tetroxide, are hypergolic and have significant toxicity, thus limiting their use to the role of propellants only. Similarly, existing N2O4, MMH, N2H4, hydrocarbons heavier than CH4, helium gas, and nitrogen propellants can not be made easily if at all from the indigenous materials on the moon or Mars.
The indigenous availability of the propellant is important because if the space vehicle is undertake longer manned flights, like those proposed to be taken to the moon and Mars, it is anticipated that there will be a need to fill the tanks of the vehicle with these propellants in their liquid form in space and fill them with materials that can be obtained at the landing sites. To accomplish this, the propellants, in their constituent form, should be liquid for ease of transfer and storage. To be a liquid, hydrogen and oxygen must be cooled to be put in a cryogenic state.
However, in low gravity, surface tension causes most liquids to adhere to and wrap around the tank wall instead of collecting at the tank “bottom.” This tendency also causes difficulties in transferring propellant, as opposed to “filling” the tank the inflowing liquid tends to cling to the sides of the tank. This also makes it hard to determine how much propellant is in a tank. Pressurizing the tank assists in filling the tank and is necessary for operating the propulsion and reaction control systems of the space vehicle. Similarly, pressurization can be utilized to transfer liquid propellants between tanks in refilling operations.
A pressurization system is commonly used in existing space vehicles to maintain the propellant tanks within preselected pressure ranges for proper main engine operation. Many propulsion systems use a separate gaseous helium system or similar separate gas system for pressurization of propellants. For example the current in-space propellants of the Space Shuttle are dinitrogen tetroxide (N2O4) and monomethylhydrazine (MMH) using a helium pressurization system. Such a system, however, is impractical for long duration space flights, such as those to the moon or Mars. The large amounts of helium needed for pressurization of such a propellant system as well as the propellants themselves are not readily available on the moon or Mars. Thus, an alternative approach to separate helium pressurization systems and other pressurization gas systems may be beneficial for spacecraft that are to be recharged at the moon or Mars.
Additionally, for these previously known pressurization options, the return propellant is typically carried from Earth thus leading to a much larger and costly vehicle. Additional oxygen and hydrogen for use with the main propulsion system might be made on the moon and Mars from water ice as well. Pressurization gases for the liquid oxygen and liquid hydrogen main propulsion tanks could generate and use the gaseous forms of the propellant, gaseous oxygen and gaseous hydrogen respectively, to pressurize and operate the space vehicle in an autogeneous system.
Thus there are significant advantages to autogenously generated pressurization gasses that are capable of being found indigenously in sufficient quantities at landing sites. Unfortunately, the transport volumes of sufficient gaseous oxygen and gaseous hydrogen is problematic in a lengthy flight space, thus the bulk of the oxygen and hydrogen will be in the cryogenically cooled liquid form. However, tank pressurization in low gravity is not straight forward. In particular, incoming pressurant gases tend to condense into the respective liquids when flowed into the cryogenic cooled liquid tanks. The low-gravity coast of the spacecraft without acceleration tends to cause the liquid propellants to coat the tank walls and generally float about the tank due to surface tension. This can lead to rapid cooling of the incoming gases and condensation of the gasses into liquids. Thus, it is difficult to predict the gaseous oxygen and gaseous hydrogen masses needed to maintain pressure in the propellant tanks due to uncertainties in the amount of condensation in the dynamic low-gravity environments.
These environments are potentially subject to a wide variety of variables, including but not limited to variable external heating, rapid cooling, gravitational interference from orbiting bodies. Due to this wide potential variability in required mass, the conventional use of engine heat to provide gaseous pressurant may be insufficient. An autogeneous system for generating pressurant gases for propulsion would need to overcome these complexities by providing a robust and active pressure management system within the propulsion system. Such a system would need to provide for rapid cooling and heating of components and actively manage tank pressures in a dynamic low-gravity environment.
Similar problems may exist in low-gravity propellant transfer situations, especially in storing the propellant, heating can cause liquid propellants in a storage tank to boil, increasing pressures. To keep the propellant liquid, one must carefully control the heat leak into the liquid and/or remove the heat by cooling. A system to keep heat from reaching the tank or to refrigerate the propellants so that they do not need to be vented to prevent pressure buildup may be preferable. Thus, any such system should be capable of being utilized on spacecraft as well as space stations.
Accordingly, it is desirable to provide a propulsion system for spacecraft that operates on or in the vicinity of the Earth's moon and Mars or where water ice is expected to exist that can provide the oxygen and hydrogen to recharge the vehicle for reuse and return, at least to some extent.